Semiconductor wafers are generally prepared from a single crystal ingot (e.g., a silicon ingot) which is then sliced into individual wafers. While reference will be made herein to semiconductor wafers constructed from silicon, other materials may be used as well, such as germanium or gallium arsenide.
One type of wafer is a silicon-on-insulator (SOI) wafer. An SOI wafer includes a thin layer of silicon atop an insulating layer (i.e., an oxide layer) which is in turn disposed on a silicon substrate. A silicon-on-insulator wafer is a type of silicon-on-insulator structure.
An example process of making an SOI wafer includes depositing a layer of oxide on a polished front surface of a donor wafer. Particles (e.g., hydrogen atoms or a combination of hydrogen and helium atoms) are implanted at a specified depth beneath the front surface of the donor wafer. The implanted particles form a cleave plane in the donor wafer at the specified depth at which they were implanted. The surface of the donor wafer is cleaned to remove material deposited on the wafer during the implantation process.
The front surface of the donor wafer is then bonded to a handle wafer to form a bonded wafer. The donor wafer and handle wafer are bonded together by exposing the surfaces of the wafers to a plasma containing, for example, oxygen or nitrogen. Exposure to the plasma modifies the structure of the surfaces in a process often referred to as surface activation. The wafers are then pressed together and a bond is formed therebetween. This bond is relatively weak, and must be strengthened before further processing can occur.
In some processes, the bond between the donor wafer and handle wafer (i.e., a bonded wafer) is strengthened by heating or annealing the bonded wafer pair at temperatures between approximately 300° C. and 500° C. The elevated temperatures cause the formation of covalent bonds between the adjoining surfaces of the donor wafer and the handle wafer, thus solidifying the bond between the donor wafer and the handle wafer. Concurrently with the heating or annealing of the bonded wafer, the particles earlier implanted in the donor wafer weaken the cleave plane. A portion of the donor wafer is then separated (i.e., cleaved) along the cleave plane from the bonded wafer to form the SOI wafer.
The bonded wafer is first placed in a fixture in which mechanical force is applied perpendicular to the opposing sides of the bonded wafer in order to pull a portion of the donor wafer apart from the bonded wafer. According to some methods, suction cups are used to apply the mechanical force. The separation of the portion of the donor wafer is initiated by applying a mechanical wedge at the edge of the bonded wafer at the cleave plane in order to initiate propagation of a crack along the cleave plane. The mechanical force applied by the suction cups then pulls the portion of the donor wafer from the bonded wafer, thus forming an SOI wafer. According to other methods, the bonded pair may instead be subjected to an elevated temperature over a period of time to separate the portion of the donor wafer from the bonded wafer. Exposure to the elevated temperature causes initiation and propagation of a crack along the cleave plane, thus separating a portion of the donor wafer.
The resulting SOI wafer comprises a thin layer of silicon (the portion of the donor wafer remaining after cleaving) disposed atop the oxide layer and the handle wafer. The cleaved surface of the thin layer of silicon has a rough surface that is ill-suited for end-use applications. The damage to the surface may be the result of the particle implantation and the resultant dislocations in the crystal structure of the silicon. Accordingly, additional processing is required to smooth the cleaved surface.
To smooth and thin the surface layer of silicon (i.e., cleaved surface), previous processes used high-temperature gaseous etching (i.e., epitaxial-smoothing (epi-smoothing)) or the deposition of a thin layer of silicon on the surface layer (i.e., epitaxial-deposition (epi-deposition)). In these previous methods, the etching or deposition is carried out at temperatures where the reaction is transport limited (i.e., the rate of reaction is limited by the availability of fresh reactants). These transport limited reactions result in thickness variations (e.g., sharp gradients in the thickness profile) at the edges of the surface layer of silicon. Further processing is needed to eliminate the thickness variations caused by the previous processes. Previous attempts to reduce the thickness variations at the edges of the surface layer have involved stripping the exposed oxide layer from the handle wafer. However, stripping the oxide layer from the handle wafer is time-consuming and costly and often results in significant bowing or warping of the wafer due to the residual stresses caused by the unexposed portion of the oxide layer.
Thus, there remains an unfulfilled need for a wafer surface treatment method that addresses the disadvantages of current treatment operations and is suitable for use in bonded wafer processing operations.